The introduction of fiber-based optical communication has brought a great increase of long haul telecommunications: the inherent low cost, wide bandwidth and small attenuation of fibers are key factors in prevailing over copper wire. In short haul access network, however, fiber qualities are superseded by the current high costs of optical transceivers. These components are usually hybrid assembly of III-V devices such as lasers, modulators, photodiodes and waveguide.
In order to succeed, novel approaches for transceiver fabrication are required. Among others, silicon-based opto-electronics is attractive because of its potential low cost, scalability and reliability and integration with the mature and unsurpassed silicon VLSI technology.
In addition, light detection in the near-infrared (NIR) region is of extreme importance in optical telecommunications, particularly when high bit rates and low power levels are involved. It becomes therefore crucial to employ NIR detectors that not only exhibit good sensitivity and speed in the spectral range on interest, but that can be closely interconnected to the to the driving/biasing and amplifying electronic circuits. Since the most common platform for electronic processing of signals is based on silicon, the integration of NIR photodiodes on standard silicon platform has been pursued in the past two-decades as a viable low-cost and high-efficiency solution to the growing request for compact semiconductors microsystems for optical signal processing.
Several approaches have been proposed, such as hybrid integration of III-V based devices (silicon optical bench SIOB) or monolithic integration of InGaAs on silicon.
Silicon-germanium (SiGe) has been considered a promising alternative to InGaAs, due to its large absorption coefficient in the NIR and good carrier transport properties. However, due to its relative lattice mismatch, the epitaxy of SiGe requires the use of appropriate buffer layers or other techniques which possibly hinder a seamless integration with CMOS Si-electronics. Nevertheless, a number of successful attempts have been reported to date.
In “Si-based Receivers for Optical Data Link”, written by B. Jalali and published in Journal of Lightwave Technology, vol. 12, no 6, of June 1994, pages 930-935, a GexSi1-x waveguide pin detectors grown by rapid thermal chemical vapour deposition is presented. A typical device structure consists of a GexSi1-x/Si multiple quantum well absorption layer, Si cladding layers approximately 1 μm thick, and n+ and p+ contact layers.
One of the most appealing attempts of designing NIR photodetectors that can be integrated with standard semiconductor technology is based on polycrystalline Ge mainly because of the low thermal budget required in the device fabrication. Polycrystalline films are deposited at low temperatures which guarantee a good compatibility with standard Si processing. The deposited films exhibit absorption spectra similar to those of monocrystalline Ge, but mobility and lifetimes are reduced.
In “Monolithic integration of near-infrared Ge photodetectors with Si complementary metal-oxide-semiconductor readout electronics” written by G. Masini et al. and published in Applied Physics Letters, volume 80, no 18, of May 2002, pages 3268-3270, a monolithic integration of an array of near-infrared Ge photodiodes on Si complementary metal-oxide-semiconductor (CMOS) electronics is reported. The chip, realized with standard very large scale integrated silicon techniques, hosts eight CMOS switches used to select one out of eight pixel. The photocurrent generated by the selected photosensitive pixel is fed to a transimpedance amplifier with an external feedback resistor R. The output is a voltage corresponding to the light intensity at the interrogated detector in the linear array. The latter encompasses eight poly-Ge/Si heterojunction photodiodes, each of them realized between the evaporated poly-Ge and the n-well left exposed in the Si substrate. Si areas are connected to metal pads. Additionally, silver was evaporated and lithographically defined in the shape of thin fingers on the central portion of the poly-Ge, to lower the series resistance.
In “2.5 Gbit/s polycrystalline germanium-on-silicon photodetector operating from 1.3 to 1.55 μm” written by G. Masini et al. and published in Applied Physics Letters, volume 82, no 15, of April 2003, a fast polycrystalline germanium-on-silicon heterojunction photodetector for the near-infrared is described. The poly-Ge-on-Si photodiodes are fabricated by thermal evaporation of germanium on a silicon n-type substrate held at a temperature of 300° C. The thickness of the resulting Ge film is 120 nm. After deposition, the detector area is defined by wet etching of square mesas 200×200 μm2 in size. The deposition and lithographic definition of Ag contacts complete the fabrication. Light is coupled to the active detector area through the substrate, transparent at the wavelength of interest. Therefore this device is fabricated for normal incidence detection and it exhibits a 1.3 μm responsivity of 16 mA/W, dark current below 2 mA/cm2 and an operational speed higher than 2.5 Gbit/s.
Applicants attribute the low responsivity of this device to the small active region (i.e., depletion layer plus one diffusion length), which can be typically of 50 nm in Ge on Si, associated to the large acceptor-like defect-density typical of polycrystalline germanium. Applicants believe that an increase in responsivity is expected in waveguide geometry where the absorption efficiency depends on the detector length rather than on the thickness of the active layer (such as in normal incidence detectors).
In “Near-infrared waveguide photodetector based on polycrystalline Ge on silicon-on-insulator substrates”, written by G. Masini et al., published in Optical Materials 17 (2001), pages 243-246, an integration of a poly-Ge photodetector with a waveguiding structure is depicted. This approach allows the distributed absorption of the incoming light in the thin sensitive layer of the poly-Ge/Si heterojunction, thus increasing the effective absorption length and efficiency. Bond and Etch-back Silicon-on-Insulator substrates with 2 μm thick n-type silicon and 1.5 μm thick SiO2 insulator are chosen as substrates. Polycrystalline Ge films were grown by thermal evaporation using a 99.999% purity commercial source. Film thickness is selected to be 120 nm. The device responsivity was measured at normal incidence (shining light from the substrate) and in the waveguide configuration. In both cases a semiconductor laser emitting 5 mW at 1.3 μm was used. From experiments, an increase in responsivity by a factor 8 has been shown in the waveguide configuration as compared to the normal incidence.
Applicants note that in this article a planar waveguide configuration of the photodiode is used, where the light is not confined laterally. The light signal travelling in the waveguide rapidly diverges in the waveguide plane, where confinement is not present. Due to this divergence, the light intensity (Watt/cm2) decreases and it becomes necessary to increase the area of the photodetector to maintain a good efficiency. However, by increasing the photodetector area, the overall speed of the device can be reduced.
In US patent application n. 2004/0188794 in the name of Prakash Gothoskar et al., a photodetector for use with relatively thin (i.e. submicron) silicon optical waveguides formed in a silicon-on-insulator structure is disclosed. The photodetector comprises a layer of poly-germanium disposed to couple at least a portion of the optical signal propagating along the silicon optical waveguide. The silicon optical waveguide may comprise any desired geometry, with the poly-germanium detector formed to either cover a portion of the waveguide or be butt-coupled to an end portion of the waveguide.
Applicants note that in all embodiments of the cited patent application, the electrical contacts are formed at the opposite ends of the detector, i.e. they are both in contact with the poly-germanium layer. In addition, the poly-Ge layer comprises a p-i-n structure, having a p-doped poly-germanium layer, an intrinsically doped layer and an n-doped layer. Applicants have observed that a p-i-n structure in poly-Ge is difficult to realize, especially with deposition technologies that employ relatively low temperatures. Relatively low deposition temperatures, i.e., not larger than 350-400° C., are desired in order to preserve compatibility with standard silicon CMOS technology.